1. Field of the Invention
This invention relates to a schaukasten for observing an image forming film through utilization of light passing through a light diffusing plane, and more particularly to a thin schaukasten which is formed by a diffusive transparent plate, a light homogenizing member, a fluorescent lamp or lamps and a reflection member.
2. Description of the Prior Art
Generally, in a schaukasten for observing an image forming film using light passing through a light diffusing plane, the distribution of luminous flux in the light diffusing plane is required to be uniform. A method that has heretofore been employed to meet this requirement is to increase the distance between a fluorescent lamp and the light diffusing plane; however, this inevitably increases the thickness of the schaukasten. Accordingly, the conventional schaukasten is bulky and requires a large dead space for its installation, and such schaukasten is heavy and hence is difficult to carry. Further, a large distance between the fluorescent lamp and the light diffusing plane leads to the defect of increased loss of light.
To overcome such defects, there has also been proposed to reduce the thickness of the schaukasten by increasing the number of fluorescent lamps used, using a diffusive transparent plane of a high diffusion coefficient or using a special light source such as a neon lamp or electroluminescence device. However, the schaukasten reduced in thickness by such method has the defects of high manufacturing cost, high running cost, heavy weight, complexity in the operation for exchanging the light source and unnecessarily high brightness which imposes marked eye-strain on the user.
In a schaukasten having incorporated therein fluorescent lamps, since the lighting frequency for the fluorescent lamp is the same as the frequency of the commercial power source, flicker is visible in the vicinity of the fluorescent lamp. Especially when the fluorescent lamp used is small-sized or near the end of its useful life, the flicker is remarkable to give the user an unpleasant feeling and to impose heavy eye-strain on him.
Further, the schaukasten having incorporated therein fluorescent lamps calls for one lighting circuit for each lamp, and hence is expensive and heavy. The cost of the schaukasten of this type is markedly increased by the provision of a frequency converter.
There has not been proposed a thin schaukasten which is composed of a diffusive transparent plate, a light homogenizing member, a fluorescent lamp or lamps and a reflection member and in which the lighting circuit for the fluorescent lamps includes a frequency converter and is used in common to the fluorescent lamps.